As electronic devices increasingly become portable, advances must be made in energy storage systems and energy conversion devices to enable such portability. Indeed, it is often the case with current electronic technology that the limiting factor to portability of a given device is the size and the weight of the associated energy storage device. A small energy storage device, such as a battery, may be fabricated for a given electrical device but at the cost of energy capacity. Conversely, an energy source with long operating time between recharging can be built, but it is often too large or too bulky to be comfortably portable. The result is that the energy source is either too heavy or does not last long enough for a particular user's application.
Numerous different battery systems have been proposed for use over the years. Early rechargeable battery systems included lead acid, and nickel cadmium (Ni-Cd), each of which has enjoyed considerable success in the market place. Lead acid batteries are preferred for applications in which ruggedness and durability are required and hence have been the choice of automotive and heavy industrial settings. Conversely, Ni-Cd batteries have been preferred for smaller portable applications. More recently, nickel metal hydride systems (NiMH) have found increasing acceptance for both large and small applications. Still other new secondary batteries have appeared with high voltage, low weight, high energy density, and long duty life, particularly the lithium ion rechargeable cells.
Notwithstanding the success of the foregoing secondary battery systems, the greatest demand for these battery systems is in applications where the battery must be able to be recharged repeatedly and safely. For traveling end-users, especially in remote areas or marine locations, it is not always feasible to find an electrical outlet to recharge the battery. Replacement costs of batteries and a paucity of retail battery outlets may make it impractical to replace the transmitters. Even where local power sources are available, some travelers have found it necessary to carry a variety of battery chargers to accommodate differences in voltage, amperage, and frequency in various parts of the globe.
One solution to the difficulty of recharging batteries in remote locations or where rechargeability is not practical would be to replace or supplement the battery pack with a photovoltaic or solar energy conversion device. With this solution, the user then has a portable "power plant" and can thus minimize his or her dependence on "plug in" power. Solar power is considered to be an environmentally clean technology, and the energy, as long as there is sunlight, is essentially free for the taking. Also, photovoltaic modules enjoy a reputation for a long working life, and thus may last the useful life of the portable equipment in which they are employed.
Photovoltaic efficiencies of conversion into electrical energy for best commercially available cells today are at 17% conversion. Ideally, for instance, a 1-cm.sup.2 single crystalline silicon cell (i.e., the most efficient variety) may provide around 35 mA current under full sun (direct sun at noon on a bright summer day). In reality because of efficiency losses, the realized current is much lower. Of the commercially available photovoltaic cells, single crystalline silicon cells are the most efficient, followed by polycrystalline, and then amorphous. The use of solar cells are intended for applications where sunlight is readily available. Under conditions of low light intensity the use of solar cells is simply not suitable.
In some portable communications, a portable solar module may provide the required current where current drain requirement is low. However, digital technology, which is a fast growing market and the prevalent profile for many of the new communication products, requires high current drains under the pulse discharge.
For digital transmission applications with their typically high pulse drain currents, a solar module of the conventional type used today would need to be impracticably large.
Accordingly, there exists a need for solar powering devices capable of high current outputs.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic representation of a GSM radio frequency telecommunication digital pulse profile.
FIG. 2 is a schematic representation of an Iridium satellite radio frequency telecommunication digital pulse profile.
FIG. 3 is a schematic representation of a circuit incorporating the present invention.
FIG. 4 is a schematic representation of a telecommunication digital pulse discharge profile (410 and 420) superimposed on profiles of output and input currents carried by a solar module and a capacitor in parallel electrically according to the present invention. The standby phase of the profile is illustrated by a horizontal solid line in the digital discharge profile (410). The dotted line (430) represents output capability from the solar module under operating conditions: stippled blocks (440) represent the solar module's capability to contribute to the power spike demanded by the device. The unshaded blocks (420) represent the contribution of the discharging capacitor to the power spike demanded by the digital device. The intermittent dash horizontal line (450) represents zero current baseline. The dashed lines (460) represent the capacitor re-charging cycle. The electrical current from the solar module that is not used either to supply a fraction of a power spike directly or to recharge the capacitor is drained off or is cut off by a switch.